This invention relates to oil and gas drilling, and more particularly to apparatus and methods for reliably transmitting information along downhole drilling strings. In the downhole drilling industry, MWD and LWD tools are used to take measurements and gather information with respect to downhole geological formations, status of downhole tools, conditions located downhole, and the like. Such data is useful to drill operators, geologists, engineers, and other personnel located at the surface. This data may be used to adjust drilling parameters, such as drilling direction, penetration speed, and the like, to accurately tap into oil, gas, or other mineral bearing reservoirs. Data may be gathered at various points along the drill string. For example, sensors, tools, and the like may be located at or near the bottom-hole assembly and on intermediate tools located at desired points along the drill string.
Nevertheless, data gathering and analysis represent only certain aspects of the overall process. Once gathered, apparatus and methods are needed to rapidly and reliably transmit the data to the earth's surface. Traditionally, technologies such as mud pulse telemetry have been used to transmit data to the surface. However, most traditional methods are limited to very slow data rates and are inadequate for transmitting large quantities of data at high speeds.
In order to overcome these limitations, various efforts have been made to transmit data along electrical or other types of cable integrated directly into drill string components, such as sections of drill pipe. In such systems, electrical contacts or other transmission elements are used to transmit data across tool joints or connection points in the drill string. Nevertheless, many of these efforts have been largely abandoned or frustrated due to unreliability and complexity.
For example, one challenge is effectively integrating a transmission line into a downhole tool, such as a section of drill pipe. Due to the inherent nature of drilling, most downhole tools have a similar cylindrical shape defining a bore. The wall thickness surrounding the bore is typically designed in accordance with weight, strength, and other constraints imposed by the downhole environment. In some cases, milling or forming a channel in the wall of the downhole tool to accommodate the transmission line may excessively weaken the wall. Thus, in certain embodiments, the only practical route for the transmission line is through the bore of a downhole tool.
Nevertheless, routing the transmission line through the bore may expose the transmission line to drilling fluids, cements, wireline tools, or other substances or objects passing through the bore. This can damage the transmission line or cause the transmission line to interfere with objects or substances passing through the bore. Moreover, in directional drilling applications, downhole tools may bend slightly as a drill string deviates from a straight path. This may cause the transmission line to deviate away from the inside surface of the bore, thereby worsening the obstruction within the bore.
Thus, apparatus and methods are needed to protect the transmission line, routed through the bore of a downhole tool, from drilling fluids, cement, wireline tools, or other components traveling through the bore.
Further, apparatus and methods are needed to maintain a transmission line against the inside surface of the bore even when the downhole tool bends or deviates from a linear path.
Further, apparatus and methods are needed for lining the inside surface of the bore to isolate a transmission line from objects or substances traveling through the bore.
Further, when dissimilar materials having varying electrical potentials are used, and in some cases when similar materials are used, mechanisms may be needed for protecting the bore wall of the downhole tool from the electrical potential of the apparatus for isolating the transmission line, the apparatus for maintaining the transmission line against the inside surface of the bore wall, and the apparatus for lining the inside surface of the bore wall.